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    <title>Document</title>
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    <script>
        console.log(a)   // 变量在定义前使用是 undefined 就是因为 变量提升
        var a=10

        // 浏览器解析为
        var a = undefined
        console.log(a)
        a=10

        console.log(0.1+0.2);   
        console.log(0.4-0.1);
        console.log(1-0.7);   
        console.log(0.7-0.4);   
           
    </script>
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